Drilling fluids, of the general type disclosed in this application, are used in drilling oil and gas or water wells to transport drill cuttings to the surface of the well, to control formation pressures, maintain bore hole stability, protect productive formations, and cool and lubricate the bit and drill string. In order to accomplish these purposes, the drilling fluid must be viscous enough to carry away the cuttings while drilling is in progress, and to prevent the well from caving. It must be temperature-stable so as not to settle or lose viscosity at the high temperatures generated while drilling is in progress. Its gel strength, or thixotropic properties, must be such that when drilling is discontinued, cuttings remain in suspension, but at the same time, the mud should not gel so much that drilling becomes difficult to start up again. In addition, the drilling fluids should form a filter cake on the sides of the bore hole such that fluid loss from the drilling fluid into the surrounding formation is minimized to prevent damage to the formation. Further, the mud must be responsive to the addition of standard fluid loss or filtration control additives, thinners, and other chemicals typically added to change its properties when such changes become necessary during drilling.
Saltwater muds are useful in connection with off-shore drilling activities, and in connection with remote on-shore drilling activities especially where salt domes are encountered. Salt normally tends to flocculate fresh water muds and cause them to settle out. However, salt contamination from a salt dome does not so affect muds made from salt water.
There are two standard procedures for preparing saltwater muds. The first is to mix attapulgite clay with the saltwater together with a thinner and a filtration control agent. The preparation of this type of drilling mud is described in "Principles of Drilling Fluid Control" (Subcommittee of the API Southern District Study Committee on Drilling Fluids, ed.), 12th ed. 1969, at Page 79.
The attapulgite particles have a needle-like structure unlike bentonite clay particles which are composed of small platelets. Therefore the viscosity-building properties of attapulgite depend on the enlargement of the needle-like particles. The unorganized meshing of the attapulgite particles accounts for the poor filtration properties of this clay, as needle-like structures do not favor the formation of a thin, impermeable filter cake. Thus water from the mud is able to enter and weaken the surrounding formation.
A second type of saltwater mud may be prepared from bentonite clay which has first been hydrated with fresh water. A process for preparing such a mud is described in U.S. Pat. No. 3,360,461, and the "Baroid Petroleum Services Mud Technology Handbook," 1965, at Page 69. In the latter reference, the bentonite clay is hydrated with fresh water, followed by the addition of a thinner and caustic soda. This mixture is then added to saltwater at a ratio of 1 part of the mixture to 3 parts of the saltwater. The addition of the premix to the saltwater generates foam so that the use of a deformer such as octyl alcohol is required. A more important drawback to this method is the necessity for transporting large quantities of fresh water to off-shore drilling locations to hydrate the clay.
Other types of mud systems employed in seawater include caustic-lignite mud, lime mud, and gypsum mud. Lignosulfonate muds prepared with attapulgite clay have all but replaced other types of seawater muds. Pages 63 through 67 of the above referenced "Mud Technology Handbook" describe typical lime and gyp muds, and a description of lignosulfonate muds is given on Pages 79 through 80 of the above referenced "Principles of Drilling Fluid Control." U.S. Pat. No. 3,985,667 also describes a lignosulfonate mud containing hydroxy acids and an iron complex.
Muds are ordinarily classified as saltwater muds when they contain over 1% salt (6000 ppm of chloride ion) and have not been converted to another type of mud, such as lime mud or a lignosulfonate mud.
Past attempts to hydrate bentonite clay with saltwater of over 6000 parts per million chloride concentration have resulted in settling out of the clay and development of viscosities of only a small percentage of those which would result if fresh water were used. The use of hard water such as water containing over about 2000 mg/l dissolved minerals to hydrate bentonite clays has likewise resulted in failure to obtain desired yield and unsatisfactory viscosities, due to the lessening of interparticle forces between the bentonite particles causing flocculation. Gel strength and filtration rate are also adversely affected by dissolved electrolytes in saltwater and hard water.
Further, because prior art methods of preparing saltwater bentonite drilling muds have required prehydration of the clay with fresh water, as well as prehydration of the thickening agent, usually followed by the addition of other ingredients, the use of pre-packaged mixtures of pre-measured dry ingredients has not been possible.
Accordingly, a method for hydrating bentonite with readily available saltwater or hard water, as opposed to fresh water, is needed. It is further desirable to provide a pre-packaged dry mix for hydration with saltwater or hard water for the convenience of the user, to obviate individual operator-caused variations in the quality of the drilling mud.